An inter-frame difference method and a background difference method are known as prior art for detecting an in-image object. For example, in the object detector configured according to the block diagram shown in FIG. 10, a camera 1011 images an object and background, and an A-to-D converter 1012 converts the analog signal of the image captured from the camera 1011 into a digital signal.
A present input image accumulating section 1013 accumulates present images sent from the A-to-D converter 1012, that is, present image inputs. A preceding input image accumulating section 1014 accumulates images of preceding frames obtained before the present input image sent from the present input image accumulating section 1013, that is, preceding input images. A background updating section 1015 updates the background using a predetermined background update procedure. A background image accumulating section 1016 accumulates the background images generated in the background updating section 1015.
A time difference and binarizing section 1017 calculates the difference between the present input image sent from the present input image accumulating section 1013 and the preceding input image sent from the preceding input image accumulating section 1014, that is, performs differential processing. The time difference and binarizing section 1017 uses a predetermined threshold value to binarize the image signal that has undergone the differential processing into 1 or 0. A background difference and binarizing section 1018 calculates the difference between the present input image sent from the present input image accumulating section 1013 and the background image sent from the background image accumulating section 1016. The background difference and binarizing section 1018 uses a predetermined threshold value to binarize the image signal that has undergone the differential processing into 1 or 0.
A moving object area image extracting section 1019 extracts the area that includes only a moving object from the whole image area. A moving object area image accumulating section 1020 binarizes the image from the moving object area extracting section 1019 and stores the binarized image. A still object area image extracting section 1021 extracts the area that includes only a still object from the whole image area. A still object area image accumulating section 1022 binarizes the image from the still object area extracting section 1021 and stores the binarized image.
Furthermore, a detection result judging section 1023 judges whether or not the detected still object is false based on the moving object area image sent from the moving object area image accumulating section 1020 and the still object area image sent from the still object area image accumulating section 1022. A judged still object area image accumulating section 1024 accumulates two-dimensional images, each of which is image data obtained by adding the judgment result from the detection result judging section 1023 to the still object area. Finally, an event sensing section 1025 receives the judgment result from the detection result judging section 1023 and uses a predetermined procedure to sense an event within the imaging region of the camera 1011.
In the prior art, by thus combining the frame difference and the background difference, a moving object and a still object are detected. In the prior art, the background image is updated only for the area where no object is present based on the detection result. Such prior art is disclosed in Japanese Patent Laid-Open No. 2002-150295 (pages 2 to 8, FIG. 1).
However, in the conventional object detection method, the background difference is used to detect a still object. Thus, a false object detection result will not correctly update the background, which disadvantageously affects the following detection result.
The present invention has been made in view of the above circumstances. An object of the present invention is to provide an object detector capable of accurately detecting a still object without using background difference.